Fix schema-v2 platform conformance producer checkout drift

[Copilot]

The platform-conformance (linux-x64) job was failing in phase1.stage.evidence-authority.checkout because schema-v2 conformance was validating the current workflow checkout as the evidence producer, while the SDK validator expects the reviewed Chat producer commit recorded in its compatible producer entry.

• Producer checkout source

  • Change schema-v2 conformance to load the SDK validator before cloning the producer checkout.
  • Derive the reviewed Chat producer identity from assertEvidenceProducerCompatibility(...).
  • Clone that reviewed producer commit instead of trusting workflow HEAD.

• Authority verification order

  • Move producer checkout creation after validator loading and lock compatibility checks.
  • Keep the existing distinctness/identity verification, but run it against the validator-approved producer checkout.

• Frozen conformance metadata

  • Refresh governed byte pins for:
    • scripts/phase1-schema-v2-producer.mjs
    • .github/workflows/client-v1-conformance.yml
    • phase1-conformance.lock.json
    • src/phase1-conformance-lock.test.ts
    • docs/phase1-conformance.md

• Focused regression coverage

  • Update the conformance test to assert that schema-v2 producer checkout selection now follows the validator-approved reviewed Chat revision rather than the triggering workflow checkout.

const identity = reviewedSchemaV2ProducerIdentity(sdkContract);

await cloneProducerCheckout(
  executionRoot,
  options.chatSourceRoot,
  sdkContract,
  environment,
);

[BunsDev]

Closing: producer resolution is already explicit. #314 added the producer_revision dispatch input with an ancestry gate, and protected runs are dispatched with the revision the SDK validator binds (for example run 35566636457 with producer ac1c4f4). The failures these branches chase were a MAX_PATH overflow in aws-lc-sys on Windows, diagnosed on #219 (issuecomment-5757703171) and fixed in #348. Defaulting the producer from the SDK lock would also change a governed workflow outside the repin process.